The present invention relates to a method of refining glyceride oil, and in particular to such a method comprising a neutralization treatment in which alkali is mixed into crude or water degummed glyceride oil and a separation treatment in which the soapstock so formed is separated from the glyceride oil.
Glyceride oils of in particular vegetable origin, such as soybean oil, rapeseed oil, sunflower oil, safflower oil, cotton seed oil and the like, are valuable raw material for the food industries. These oils in crude form, usually obtained from seeds and beans by pressing and/or solvent extraction, contain several compounds other than triglycerides. Some of these compounds such as phosphatides, free fatty acids, odours, colouring matter, waxes and metal compounds must be removed because they adversely affect taste, smell, appearance and keepability of the refined oil.
In general, the first step in the refining of glyceride oils is the so-called degumming step, i.e. the removal of phosphatides. In conventional degumming processes water is added to the crude glyceride oil at a temperature ranging from 60.degree. to 90.degree. C. to hydrate the phosphatides, which are subsequently removed, e.g. by centrifugal separation.
However, most of the afore mentioned impurities, including the non-hydratable phosphatides, require a chemical treatment to remove them and refining by neutralization with alkali is generally operated to this end. Alkali refining comprises, in its broadest sense, addition of an aqueous alkali solution to crude or water-degummed oil, hydration and a separation treatment in which the soapstock thus formed is removed from the glyceride oil. The alkali-refined glyceride oil is finally washed once or twice with water to remove residual soaps that otherwise affect subsequent refining by bleaching.
Entrainment of triglyceride oil with the soapstock and with the washing water, and saponification of triglyceride oil through contact with the refining agent constitute important refining losses.
Therefore, a multitude of modifications to the original alkali refining technique have been developed in an attempt to reduce these refining losses.
The most frequently applied modification constitutes the removal of the phosphatides prior to the alkali refining to reduce emulsification of triglyceride oil in the soapstock. Another modification entails pretreatment with acid of the glyceride oil to be alkali refined. It has been found that this pretreatment assists in the removal of phosphatides and pro-oxidant metal ions, such as iron and copper. U.S. Pat. No. 2,666,074 describes the use of aqueous solutions of polybasic aliphatic acids such as citric acid and tartaric acid and U.S. Pat. No. 2,702,813 describes the use of 75 to 85% phosphoric acid at levels of 0.05 to 0.15% in the oil. Such pretreatments are found to reduce emulsification of triglyceride oil and soapstock and saponification of triglyceride oil through the buffing action of the acid.
The separation stage in the alkali refining process is the most critical one since it determines the overall yield to an even greater extent than proper pretreatment.
In continuous refining, high-speed centrifuges are used to separate the oil/reagent mixture into neutral oil and soapstock. Even under optimum conditions there can never be complete separation between neutral, soap-free oil on the one hand and soaps, phosphatides, free reagent and water on the other. In all cases, a compromise has to be made between separating the soapstock with the least amount of entrained oil and thus allowing an amount of soaps to pass along with the glyceride oil for removal in subsequent washing stages, and allowing a proportion of glyceride oil to be entrained with the soapstock to yield a neutral oil with minimum residual soap content. In case an oil with minimum residual soap content is aimed at, there is also a risk that soaps become so diluted with triglyceride oil that the resistance of the soapstock outlet drops and the soapstock and the oil phase are removed from the centrifuge at the soapstock outlet under the counter pressure at the oil phase outlet.
Washing entails mixing an amount of water into the oil phase followed by removal of this washing water from the neutral oil. Alkali solutions can be used instead of water to neutralize remaining free fatty acids or diluted acid can be use to convert the residual soaps into free fatty acids to avoid emulsification and to achieve proper separation.
These washing stages however, have the disadvantage that they may again lead to triglyceride oil losses and may cause additional pollution and/or effluent disposal problems.